The homeostasis of the erythron is accomplished to a major degree by the hormone erythropoietin, which regulates marrow activity according to tissue requirements for oxygen. This program is investigating the quantitative relationship between anemia, erythropoietin production, and erythropoiesis in normal man and in human beings with chronic bone marrow and renal disesess. Studies will be done under basal conditions, and following stimulation by injections of human erythropoietin from anemic donors or treatment with androgenic hormones. The ultimate goal is to define a rational basis for the use of androgens in the treatment of human beings with severe anemia due to bone marrow failure, and of erythropoietin concentrates in patients with anemia due to renal disease.